1. Field of the Invention
This invention relates to supported chromium oxide polymerization catalysts, processes for preparing such catalysts and olefin polymerization processes utilizing the catalysts.
2. Description of the Prior Art
It is well known that olefin polymers can be prepared using supported chromium oxide catalysts. The polymerization reactions can be carried out in hydrocarbon solutions in which the polymers are soluble or in diluents in which the polymers are insoluble whereby slurries of suspended solid polymer particles are produced. In such polymerization processes, the melt indexes of the polymers can be varied by varying the polymerization temperatures, i.e., higher reaction temperatures generally result in increased melt indexes. However, in slurry polymerization processes, the reaction temperatures which can be utilized are limited in that at higher temperatures the polymers become swollen and sticky.
Other techniques are known in the art which bring about the production of polymers of higher melt indexes in slurry polymerization processes without increasing the reaction temperatures above normal limits. For example, U.S. Pat. No. 4,384,987 to Hawley discloses a polymerization catalyst comprised of supported chromium oxide which is prepared by a process including the step of treating the catalyst with a permanganate compound. The use of such a catalyst in slurry polymerization processes results in polymers having higher melt indexes.
Supported chromium oxide catalysts have heretofore been comprised of chromium oxide and supports containing one or more of silica, titania, thoria, alumina, zirconia or aluminophosphates. The catalysts are prepared by combining the chromium oxide and the support in a known manner and then activating the catalyst. By the present invention improved supported chromium oxide catalysts and processes for preparing and using such catalysts are provided. The improved catalysts have increased polymerization activities and result in polymers of high melt indexes.